RFID tags are identification tags that can be detected and “read” by radio frequency communication. The use of this technology is growing very rapidly, and is being, or is expected to be, applied in a wide variety of applications, ranging from warehouse inventory control, shipping and receiving, shipment tracking, retail sales (e.g. supermarket check-out), building security (e.g. employee ID tags), and military asset tracking. In many of these applications, RFID tags have already, or will soon, replace barcodes as the primary method of automatic item detection and identification.
Typically, RFID technology comprises an RFID detector, which is able to transmit RF signals and detect RF responses, and at least one RFID tag, which is able to receive the RF signals from the RFID detector and respond to them. The RFID tags can be active, in that they are powered by an included battery, passive, in that they are powered by the energy of the detector's RF signal, or “mixed.” An example of a mixed RFID tag would be a tag that includes a capacitor or other short term power source that is normally discharged, but is temporarily charged by the energy of an RF query signal from the detector, after which it supplies energy to the tag for a short time while the tag responds to the RF query.
With reference to FIG. 1A, detection of an RFID tag 100 begins with transmission by an RF transmitter 106 included in the RFID detector 102 of a query signal 104. Typically, the query signal 104 is an RF wave that is amplitude modulated, for example according to amplitude shift keying (ASK). See for example B. Sklar, Digital Commuincations, Prentice Hall, 2nd edition, 2001, incorporated herein by reference.
As shown in FIG. 1B, the RFID tag 100 detects the query signal 104, and responds with the requested information 110, typically including an ID number, a date, an employee ID number, or whatever information is appropriate under the circumstances. In the case of a passive RFID tag, the detector 102 continues to broadcast an unmodulated carrier RF wave 112 while it simultaneously detects the response 110 of the RFID tag 100. The carrier wave 112 serves as the power source for the RFID tag 100 while the RFID tag 100 is replying. In such cases, the passive RFID tag 100 typically modulates the carrier wave 112 by passing the carrier wave 112 through a variable impedance, and then re-transmitting the modulated RF wave 110 back to the RF receiver 108 included in the RFID detector 102.
One of the principle advantages of RFID technology, as compared for example to barcode technology, is that RFID technology does not require that the detector be directed specifically to the tag being read. While an RFID detector 102 can be somewhat directional, and its detection range can be adjusted from a few feet to a few yards, it is not necessary that the detector 102 be focused specifically on an individual RFID tag 100. In fact, the RFID tag 100 need not be visible, so long as it is within the detection range of the RFID detector 102 and is accessible to radio waves.
With reference to FIG. 1C, while the non-directional nature of RFID technology can significantly facilitate the speed with which items are detected, for example as grocery items pass by a cash register in a supermarket, this non-directionality also presents significant challenges when more than one RFID tag 114 falls within range of the detector 102 at the same time. For example, it may be desired to detect a large number of tags attached to individual items carried by a pallet that is about to be shipped. In such a case, all of the tags 114 will respond simultaneously to an RFID query 104, and it will be nearly impossible for the detector 102 to extract any meaningful information from the resulting aggregated mixture 116 of signals that is detected by the receiver 108.
So as to address this dilemma, media access control (MAC) protocols such as ALOHA or slotted ALOHA (see B. Sklar, op. cit.) are sometimes used to minimize response collisions by requiring RFID tags to transmit and retransmit their responses at random times until a collision-free response has been received from each tag. A simplified example is presented in FIG. 2. In this example, instructions are included in the query 104 that cause each of the RFID tags 114 to select a random timeslot 200 from 1 to 5 in which to transmit its response, and to continue this process until it receives an acknowledgement that its signal has been read. For convenience, the behavior of 8 tags 202 is indicated in the figure. In the first group of five timeslots 204 tags 4, 6, and 7, by chance, respond in unique timeslots and are successfully detected. Hence, they do not re-transmit during the second group 206 of five timeslots. However, tags 1 and 3 collide during the first timeslot, and tags 2, 5, and 8 collide during the third timeslot. These five tags therefore repeat their responses in randomly chosen timeslots 200 during the second group 206 of five timeslots.
In the second group 206 of five timeslots, tags 1, 3, and 8 are received in unique timeslots and are successfully detected. However, tags 2 and 5 collide in the second timeslot, and will need to be repeated again in the third timeslot group (not shown) of five timeslots. It is apparent from this example that while protocols such as ALOHA or slotted ALOHA can provide a solution for detecting a plurality of simultaneously queried RFID tags, these protocols are extremely inefficient due to the large latency that they add to the detection process. It is conceivable that a pallet of goods in a warehouse may contain hundreds, or even thousands of individual products, each with its own RFID tag. In such a case, using current RFID tag technology, it could take several minutes for an RFID reader to query and receive each tag's information.
What is needed, therefore, is a technique for rapidly and accurately distinguishing the responses of a plurality of RFID tags when all of the RFID tags are simultaneously queried by an RFID detector.